1. Field of the Invention
The present invention relates to a cylinder head for an internal combustion engine. More particularly, the invention relates to a cylinder head having pass-through holes through which fasteners are passed for fastening the cylinder head to a cylinder block to suppress the deformation of intake port openings which are opened and/or closed by intake valves due to thermal expansion at the periphery of the intake port openings. The cylinder head can also be used to suppress the deformation of exhaust port openings which are opened and/or closed by exhaust valves due to thermal expansion at the periphery of the exhaust port openings.
2. Description of the Related Art
In an internal combustion engine, a cylinder head having provided therein intake port openings and exhaust port openings which are opened and/or closed by intake valves and exhaust valves, respectively, is a part of the engine which reaches a rather high-temperature because it is exposed to combustion gas. The peripheries of the intake port openings and the exhaust port openings also reach a rather high-temperature. On the other hand, a plurality of fastening bolt pass-through holes are formed in the cylinder head radially outwardly of the intake and exhaust port openings along the peripheries of combustion chambers. Thus, the cylinder head is fastened to the cylinder block with fastening bolts inserted through corresponding fastening bolt pass-through holes.
Then, the highly heated peripheral portions of the intake and exhaust port openings tend to thermally expand towards the peripheries thereof in a substantially uniform manner. As shown in FIG. 7, however, fastening portions are provided radially outwardly of intake port openings a and exhaust port openings b. The fastening portions are fastened by fastening bolts inserted through pass-through holes c. Therefore, when a mating surface of the cylinder head with the cylinder block is viewed from the bottom, the fastening portions restrain thermal expansions in directions along straight lines d which connect the centers of either the intake port openings a or the exhaust port openings b with the centers of the pass-through holes c and in particular thermal expansions in directions along the straight lines d in the vicinity of portions where the peripheral portions of the intake port openings a or the exhaust port openings b overlap the straight lines d. As such, the peripheries of the intake port openings a or the exhaust port openings b cannot expand uniformly, which leads to the intake port openings a or the exhaust port openings b, which are each substantially round, being deformed to form non-circular shapes. Simultaneously, thermal stress converges on areas f on the peripheries of the intake port openings a or the exhaust port openings b and in the vicinity of portions which overlap straight lines e which pass through the centers of the intake port openings a or the exhaust port openings b and intersect with the straight lines d. As a result, creep deformations are generated in the areas f due to the thermal stress, which causes the intake port openings a or the exhaust port openings b to be further deformed and non-circular in shape. Note that reference character g denotes an opening through which a spark plug faces the combustion chamber.
When the intake port openings or the exhaust port openings deform as described above, the sealing properties of the intake valve or the exhaust valve deteriorate. This causes a leakage of unburned air-fuel mixture from between the intake valves and the intake port openings, or between the exhaust valves and the exhaust port openings during a compression stroke. Then, in the event that unburned air-fuel mixture leaks from between the intake valves and the intake port openings, since fuel flows back to the intake ports, the accuracy at which the air-fuel ratio is controlled may be badly affected. In the event that unburned air-fuel mixture leaks from between the exhaust valves and the exhaust port openings, the amount of HC in exhaust gas increases, resulting in deterioration in exhaust emissions.
Then, in order to prevent the reduction in sealing properties of the intake valves or exhaust valves resulting from the deformation of the intake port openings or exhaust port openings due to thermal expansions or creep deformation in the peripheral portions of the intake or exhaust port openings, conventionally, a certain limit is imposed on the maximum combustion temperature of the internal combustion engine or the capacity of the coolant jacket in the cylinder head is increased so as to improve the cooling performance. Thus, the deformation of the intake port openings or exhaust port openings is suppressed to thereby secure the sealing properties of the intake or exhaust valves.
Limiting the maximum combustion temperature of an internal combustion engine, however, sets a limit to the output of the engine, and in the case of an automotive internal combustion engine, for example, the running performance of an automotive vehicle is limited, which incorporates an internal combustion engine whose maximum combustion temperature is limited. Thus, since limiting the maximum combustion temperature of an internal combustion engine leads to limiting the operating performance of an apparatus incorporating the internal combustion engine, an improvement thereto has been desired. In addition, the enlargement of the cooling mechanism with a view to improving the cooling performance, such as increasing the capacity of the cooling water jacket, leads to the enlargement of the cylinder head, thus limiting the degree of freedom in the layout of the internal combustion engine.
The present invention was made in view of these situations and an object thereof is to maintain a high engine output with a simple structure and to secure good sealing properties of intake or exhaust valves without the enlargement of a cylinder head being involved.
According to a first aspect of the invention, there is provided a cylinder head for an internal combustion engine adapted to be fastened to a cylinder block with the fasteners. The cylinder head includes combustion chambers, intake port openings and exhaust port openings which are opened and/or closed by intake valves and exhaust valves, respectively. Pass-through holes are also included through which fasteners are passed. Space portions are provided between the combustion chambers and the pass-through holes at positions which overlap straight lines connecting the centers of the intake port openings or the exhaust port openings with the pass-through holes when a mating surface of the cylinder head with the cylinder block is viewed from the bottom.
According to the first aspect of the invention, when the peripheries of the intake port openings or the exhaust port openings are thermally expanded, the suppression of thermal expansion by the fastening portions on the cylinder head where the cylinder head is fastened to the cylinder block with fasteners is alleviated by the space portions in the peripheries of the intake port openings or exhaust port openings. The thermal expansion is suppressed in the vicinity of the portions which overlap the straight lines when the mating surface of the cylinder head with the cylinder block is viewed from the bottom thereof. Therefore, thermal expansion is permitted and the peripheries of the intake port openings or exhaust port openings thermally expand in the directions along the straight lines. Due to this, the deformation of the intake port openings or exhaust port openings based on the suppression of thermal expansion by the fastening portions on the cylinder head is suppressed. Moreover, the convergence of thermal stress occurring by the suppression of thermal expansion is reduced in the areas near the portions of the peripheries of the intake port openings or exhaust port openings which overlap the straight lines passing through the centers of the intake port openings or exhaust port openings and intersecting with the straight lines substantially at right angles. Thus, the intake port openings or exhaust port openings are allowed to maintain shapes close to their substantially round shapes provided before the peripheries of the intake port openings or exhaust port openings are thermally expanded.
As a result, even when the peripheries of the intake port openings or exhaust port openings are thermally expanded, with the simple structure in which the space portions are provided in the cylinder head, good sealing properties of the intake valves or exhaust valves can be obtained. This suppresses the leakage of unburned air-fuel mixture into the intake port openings during compression strokes, wherein the accuracy at which the air-fuel ratio is controlled can be properly maintained. In addition, similarly, the above-described structure suppresses the leakage of unburned air-fuel mixture into the exhaust ports during compression strokes, wherein exhaust emissions can be improved. Moreover, combustion at as high a maximum combustion temperature as the thermal expansion is permitted by the space portions is possible, so that high engine output can be maintained. Furthermore, there is no risk that the cylinder head is enlarged, so no limitation is imposed as to the layout of the internal combustion engine.
According to a second aspect of the invention, since the degree of alleviation of the suppression of thermal expansion by the fastening portions on the cylinder block can be substantially equalized on sides of the straight lines, the peripheries of the intake port openings or exhaust port openings can be thermally expanded more uniformly. Thus, the shapes of the intake port openings or exhaust port openings can be maintained to those which are closer to the substantially round shapes. As a result, in addition to the effectiveness provided by the first aspect of the invention, better sealing properties of the intake valves or exhaust valves can be secured. Thus, the accuracy at which the air-fuel ratio is controlled can be properly maintained and the exhaust emissions can be further improved.
According to a third aspect of the invention, there is provided a cylinder head for an internal combustion engine, as set forth in the first and second aspects of the invention, wherein the space portions constitute coolant passageways.
According to the third aspect of the invention, the suppression of thermal expansion by the fastening portions on the cylinder head is alleviated by the coolant passageways which are constituted by the space portions. At the same time, the peripheries of the intake port openings or exhaust port openings are cooled with coolant flowing through the space portions. As such, the thermal expansions in the peripheries of the intake port openings or exhaust port openings are reduced. Therefore, since the deformation of the intake port openings or exhaust port openings, and the occurrence of the convergence of thermal stress are further suppressed, the shapes of the intake port openings or exhaust port openings can be maintained to those closer to their substantially round shapes provided before the peripheries of the intake port openings or exhaust port openings are thermally expanded.
As a result, in addition to the effectiveness provided by the first and second aspects of the invention, with the simple structure in which the coolant passageways are constituted by the space portions, better sealing properties of the intake valves or exhaust valves are obtained, the accuracy at which the air-fuel ratio is controlled is better maintained, and exhaust emissions are better improved.